Support -F and --form option

[alamirault]

This PR allow to convert curl to php with options -F and --form.
Example:

curl -X POST https://api.easypost.com/v2/shipments \
     -u API_KEY: \
     -F 'field[0]=value1' \
     -F 'field[1]=@myFile.png'

is converted as

$ch = curl_init();

curl_setopt($ch, CURLOPT_URL, 'https://api.easypost.com/v2/shipments');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_POST, 1);
$post = array(
    'field[0]' => 'value1',
    'field[1]' => '@' .realpath('myFile.png')
);
curl_setopt($ch, CURLOPT_POSTFIELDS, $post);
curl_setopt($ch, CURLOPT_USERPWD, 'API_KEY' . ':' . '');

$result = curl_exec($ch);
if (curl_errno($ch)) {
    echo 'Error:' . curl_error($ch);
}
curl_close($ch);

If this PR is merged, issue #26 can be closed

[r-akshat]

Hi @incarnate, Are you planning to merge this issue anytime soon ?

[incarnate]

Sorry for the slow review, looks good, thanks for the contribution @alamirault